1. Field of the Invention
This invention relates to a hexagonal Z type magnetic oxide sintered material having a high resistance, a method for preparing the same, and an impedance device using the same.
2. Prior Art
In accordance with size and weight reductions of modern electronic equipment, a shift of the operating frequency to a higher one is now being undertaken. It is essential to tailor components of electronic equipment so as to comply with the higher frequency. Under such circumstances, there is a tendency that the noise generated by the electronic equipment is also increased in frequency.
Since the noise generated can affect the components and peripheral equipment to cause malfunction, a countermeasure for noise is essential for electronic equipment.
One countermeasure for noise is an impedance device or impedor using ferrite material.
The impedance device is to remove noise by absorbing noise as a loss of ferrite and converting it into heat. Because of simple structure and low cost, impedance devices are used in many equipment.
Prior art impedance devices mainly use nickel base spinel type ferrite. However, in conjunction with the recent shift toward higher frequency, the noise generating above the frequency band that impedance devices using nickel base spinel type ferrite can remove becomes serious.
For example, current personal computers operate at a clock frequency of higher than 100 MHz and a future shift toward higher frequency is expected. In such a case, the frequency of noise generated therefrom is higher than the clock frequency, for example, higher than 500 MHz. The spinel type oxide magnetic materials, however, have the nature that magnetic permeability suddenly drops beyond a certain frequency. Then conventional impedance devices using spinel type oxide magnetic material are quite difficult to remove noise generated at 500 MHz or higher.
Therefore, a material which is magnetic, but experiences no drop of magnetic permeability in the high frequency range is desired as the material for impedance devices. One magnetic material meeting this requirement is, for example, the hexagonal oxide ferromagnetic material disclosed in JP-B 736/1958. An impedance device using such a hexagonal oxide ferromagnetic material is disclosed in JP-A 16910/1991.
This hexagonal oxide ferromagnetic material is a hexagonal Z type magnetic oxide sintered material.
The hexagonal magnetic oxide sintered material is classified into M type (AFe.sub.12 O.sub.19), W type (AB.sub.2 Fe.sub.16 O.sub.27), Y type (A.sub.2 B.sub.2 Fe.sub.12 O.sub.22), Z type (A.sub.3 B.sub.2 Fe.sub.24 O.sub.24), etc. The Z type material is a compound represented by the general formula: M.sub.3 Me.sub.2 Fe.sub.24 O.sub.41 wherein M is an alkaline earth metal ion and Me is a divalent metal ion. Since the Z type materials wherein Me is cobalt have an axis of easy magnetization in a plane of a hexagonal crystal and hence greater anisotropy, they retain high magnetic permeability up to a higher frequency region as compared with the spinel type oxide magnetic materials and are thus best suited as the material for those impedance devices used in the high frequency region.
The hexagonal Z type magnetic oxide sintered material, however, has the drawback that its resistivity is low. In constructing an impedance device, electrolytic plating is carried out so as to cover electrodes in order to improve the reliability of the electrodes. If the resistivity of magnetic material is low, there is a likelihood that the plating metal extend from the electrode region to the magnetic material region to incur a shortcircuit accident. Probable countermeasures are masked plating and electroless plating, for example. If these countermeasures are taken, an extra step is added to the manufacture process to increase the cost of manufacture.
It is generally believed that such a problem may be avoided if the resistivity is 10.sup.6 .OMEGA.-cm or higher.
The hexagonal Z type magnetic oxide sintered material also has the drawback that its sintered density is low. Because of the current tendency that many electronic equipment parts are replaced by surface mount parts, parts are required to have mechanical strength. Impedance devices are not an exception and the mechanical strength of impedance devices themselves must be increased. Since the hexagonal Z type magnetic oxide sintered material, however, has a low sintered density, sufficient mechanical strength is seldom achievable. In general, a sintered density of at least 4.6 g/cm.sup.3, preferably at least 4.7 g/cm.sup.3 is believed sufficient to provide the strength necessary for magnetic oxide materials. Also, the sintered density is closely related to magnetic permeability so that a lower sintered density leads to a lower magnetic permeability. Then, the inherent characteristics that the hexagonal Z type magnetic oxide sintered material possesses cannot be reflected if the sintered density is low. It is known that in order to increase the sintered density of hexagonal Z type magnetic oxide sintered material, elevating the firing temperature is generally effective.
However, the hexagonal Z type magnetic oxide sintered material prepared by firing at elevated temperature contains Fe.sup.2+ which is generated as a result of reduction of Fe.sup.3 + in the magnetic oxide. The generation of Fe.sup.2+ causes to reduce the resistivity of the magnetic oxide, giving rise to a problem upon electrolytic plating as mentioned above and leaving an increased likelihood of electric breakdown.
For this reason, it is undesirable to increase the sintered density of hexagonal Z type magnetic oxide sintered material by elevating the firing temperature.
A method for preparing a hexagonal z type magnetic oxide sintered material by hot pressing was recently proposed. With this method, a high density is obtained at a relatively low temperature. This method, however, has not been practically implemented since it requires a large size installation and complex handling.